Is it possible to create a crystal which will wait until it sees the sunlight, and then start absorbing the moisture in the air? This is a concept behind an interesting new discovery in the field of materials science developed by chemists from the University of Iowa. Namely, the researchers created a metal-organic crystal in which ultraviolet light provokes a rearrangement of atoms in its lattice, thus creating cavities capable of capturing water molecules. The attractiveness of such a concept does not lie only in the fact that the material absorbs moisture but also in the deliberate light-induced activation of the process, rather than pump-driven, compressor-driven or mechanical one.
“We have found and validated a way to capture and to store water that would require only sunlight,” says Leonard MacGillivray, adjunct professor at the Department of Chemistry and the former head of the department. “You can transport the crystal lattice and eventually release the water on demand. That’s why it’s such an advance.”
The technology falls under the domain of the atmospheric water harvesters. This class of devices is struggling with an engineering challenge in that despite being quite abundant, water in the air is hardly accessible in arid conditions. Over the recent decades, metal-organic frameworks, or MOFs, have attracted a lot of attention as materials for air-to-water conversion owing to their high efficiency in adsorption. Thus, MOF-801 has already been proven to surpass silica gel and molecular sieves in harvesting atmospheric moisture, especially in desert conditions where direct cooling or fogging become unrealistic.
The innovation introduced by the Iowa chemists brings something new to the toolkit. Contrary to the permanently porous material used before, the crystals developed at the University are transformed in response to UV exposure, which allows water to be trapped inside cavities produced as a result of this exposure. According to graduate researcher Nevindee Samararathne Muhandiramge, the key difference here is: “The reason why we use the word ‘intelligent’ is because we’re triggering the water capture intentionally with the light” Furthermore, she said: “UV light is freely available from the sun. So, the next step would be to determine the limits of the water uptake in terms of mass percent and push that limit as far as we can.”
In the experimental setup, the crystals are able to store 5 percent of its mass in the form of water. While this rate is quite moderate compared to the mature solutions, the Iowa crystals outshine even such technologies as gel-based solar sorbent, which yielded 0.819-3.019 g of water per gram in 30%-to-90% RH range (relative humidity). Another example is the MOF-based adaptive air-to-water harvester which managed to harvest 3.52 l of water per kg per day under variable atmospheric conditions through adjustment of adsorption and desorption cycles in situ. However, the latter is based on a broader technological context and thus involves more components. That is a key aspect.
While atmospheric water harvesters are usually assessed according to bulk performance, their development is also hindered by the problems of reversibility, safety, manufacturability, and easy integration into a technical application. The researchers emphasize the possibility to self-assemble the crystals and imply that this feature can contribute to their manufacturability. At the same time, they acknowledge the significant constraint of their invention the cadmium used as a test metal is not safe for application. If it were possible to replace the material while retaining the structural rearrangement and achieving higher water intake rate, then the significance of the development would go well beyond another absorptive material.